Balanced charge waveform for transcutaneous pacing

ABSTRACT

External pacemaker systems and methods deliver pacing waveforms that minimize hydrolysis of the electrode gel. Compensating pulses are interleaved with the pacing pulses, with a polarity and duration that balance the net charge at the electrode locations. The compensating pulses are preferably rectangular for continuous pacing, and decay individually for on-demand pacing.

TECHNICAL FIELD

The present invention relates generally to medical pacemaker systems,and more particularly to devices and methods that generate pacingwaveforms for minimizing patient skin burns during long-termtranscutaneous defibrillation.

BACKGROUND

Systems are made for administering to patients pacing pulses externally.Advantageously, external defibrillators may also administer suchexternal pacing. A common electrode used to connect a heart pacer to apatient incorporates an electrically conductive, impedance-decreasinggel disposed between a flexible conductive plate and the patient's skin.The gel ensures good electrical contact between the patient and theconductive plate, and adheres the electrode to the patient's skin.During pacing, pulses are generated by the heart pacer and appliedthrough the electrodes and into the patient. Typical pacing equipmentwill commonly deliver a pacing pulse having an amplitude of up to 300volts, and a maximum current of about 0.2 amps. Such a pacing pulse maybe applied to a patient up to 170 times a minute, for periods as long as24 hours.

FIG. 1 is a diagram of a pacing waveform 10 currently used for thetranscutaneous pacing of a patient. Pacing waveform 10 is shown on agraph having a horizontal axis of time, and a vertical axis of current(in milliamps) applied to the patient. Prior art pacing waveform 10comprises a positive stimulating pacing pulse 12 having an amplitude 14and a duration t1. After application of the positive stimulating pulse,the amplitude of the waveform falls to at or near 0 for duration t2. Theperiod between application of the positive stimulating pulses is the sumof duration t1 and t2.

Application of pacing waveform 10 generates an excess of positive chargeflowing through the electrodes. The excess charge causes hydrolysis ofthe electrode gel, producing hydrogen and oxygen between the electrodeand the patient's skin. During long-term transcutaneous pacing, thegeneration of hydrogen and oxygen causes the electrode impedance toincrease and the electrode pH to change.

A problem arises when such transcutaneous pacing is long-term. Thechanges of an electrode's impedance and pH are dramatic. The hydrogenand oxygen gas, which tend to accumulate between the patient's skin andthe flexible conductive plate, and produces two primary undesirableeffects. First, the accumulation of the gases generally decreases theconductivity between the electrode and the patient. As the impedance ofthe electrodes increases, the pacer is forced to compensate by applyinga higher voltage to produce a suitable pacing current. Generally, theimpedance of the electrodes may reach such a high value that the paceris unable to generate a sufficient voltage to apply a pacing pulse. Atthis point, many pacers may stop pacing, and instead generate a “leadsoff” alarm, on the erroneous determination that the high resistance iscaused by one of the electrodes having fallen off the patient.

Second, the gas has a tendency to accumulate in pockets, causing thecurrent density in areas of the electrode to increase. The bubbles tendto insulate the conductive plate from the patient, reducing the surfacearea of the electrode in contact with the patient. If the density ofcurrent flow increases in the areas remaining in contact with thepatient, patient discomfort may result. If the current density increaseseven further, burning of the patient's skin may result. The increasingcurrent density problem is exacerbated with electrodes designed forpediatric use. Pediatric electrodes tend to be smaller and have asmaller conductive surface, yet have current flows comparable toelectrodes used for adults.

As hydrolysis occurs during long-term pacing, the pH of the electrodealso has a tendency to change. The formation of hydrogen and oxygenbubbles within the electrode cause the gel of one electrode to becomemore acidic, and the gel of the other electrode to become more basic.For children or patients with sensitive skin, the change in electrode pHcan become highly irritating to the dermic layer.

One approach to minimizing the buildup of hydrogen and oxygen gas hasbeen to construct the electrodes in a manner that minimizes the amountof gas that may accumulate. For example, U.S. Pat. No. 5,456,710entitled ‘VENTED ELECTRODE” discloses an electrode construction thatallows hydrogen and oxygen buildup within an electrode to pass through agas-permeable layer and escape from beneath the electrode. The gasgenerated by hydrolysis can therefore vent to the environment beforeaccumulating and causing the impedance or pH of the electrode to change.

While the accumulation of hydrogen and oxygen in the electrodes may beprevented by appropriately constructing the electrodes, such a solutionis only applicable in limited circumstances. The majority of pacingperformed today uses traditional, non-vented conducting pads. For thosesituations where non-vented electrodes are being used, it would beadvantageous to find an alternative technique to minimize the hydrolysisof the electrode gel such that the impedance and pH of the electrodeswould remain relatively constant over an extended period of time.

BRIEF SUMMARY

The invention overcomes the problem of the prior art. The inventionprovides external pacemaker systems and methods that deliver pacingwaveforms that alleviate the problem of the prior art, by minimizing thehydrolysis of the electrode gel, and thus prevent the resulting harmfuleffects.

The invention delivers a series of pacing pulses to provide pacing tothe patient. In addition, the invention applies a series of compensatingpulses interleaved with the pacing pulses. The compensating pulseshaving a polarity opposite to a polarity of the pacing pulses, tosubstantially compensate for a long-term charge of the pacing pulses.

Accordingly, the invention generates pacing waveforms that maintain abalanced charge in the electrodes. This minimizes the hydrolysis of theelectrode gel during long term transcutaneous pacing. The invention thusallows long-term transcutaneous pacing without fear that the pacing willbe stopped due to excessive electrode impedance. In addition, theinvention allows the devices, methods, and waveforms of the invention tobe applied using conventional electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects and attendant advantages of theinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram of a prior art pacing waveform generated by a priorart system;

FIG. 2 is a block diagram of a transcutaneous pacing system according tothe invention;

FIG. 3 is a time diagram of a general balanced charge waveformembodiment produced by the device of FIG. 2 and by methods of theinvention that is suitable for continuous pacing;

FIG. 4 is a time diagram of an embodiment of the waveform of FIG. 3 thatis preferred for continuous pacing;

FIG. 5 is a time diagram of a first alternative embodiment of thewaveform of FIG. 3 that is preferred for on-demand pacing; and

FIG. 6 is a time diagram of a second alternative embodiment of thewaveform of FIG. 3 that is preferred for on-demand pacing.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description. As hasbeen mentioned, the invention provides external pacemaker systems andmethods that deliver charge-balanced pacing waveforms that alleviate theproblems arising from the hydrolysis of the electrode gel. The inventionis now described in more detail.

FIG. 2 is a block diagram of a transcutaneous pacing system 100according to the invention. System 100 is made as is known in the art.System 100 may be implemented alone, or in conjunction with othersystems, such as defibrillation systems, etc. System 100 may be part ofwhat would be used by trained medical personnel, or untrained users in ahome or public access situation, or part of a wearable medical device,etc. System 100 also has transcutaneous pacing electrodes 110 forapplying to a patient (not shown). Pulses 120 are delivered to thepatient between the applied electrodes 110. In addition, system 100 hasa pacing module 140, which generates pacing waveforms for pacing thepatient, preferably in the long term. Module 140 may be implemented byelectronics, be driven by a processor such as by software programming,digital signal processing, etc. The invention generates an aggregatewaveform that includes both pacing pulses and compensating pulses, andapplies them as pulses 120 through the pacing electrodes of FIG. 2. Bothtypes may be generated by module 140.

Referring to FIG. 3, a general waveform of the invention is shown. Thereis a series of pacing pulses (150, 152, 154, . . . ,) to provide pacingto the patient, preferably in the long term. In addition, the inventionincludes delivering a series of compensating pulses (161, 163, 165, . .. ,) to substantially compensate for a long-term charge of the pacingpulses. To achieve compensation, compensating pulses (161, 163, 165,)have a polarity opposite to a polarity of pacing pulses (150, 152, 154,. . . ,). Additionally, as shown in FIG. 3, the amplitude ofcompensation pulses (161, 163, 165,) is less than the amplitude ofpacing pulses (150, 152, 154, . . . ,). The two series of pulses aregenerally interleaved. This means that some of the compensating pulsesfollow some of the pacing pulses. In the preferred embodiment, theyalternate one by one, as shown in FIG. 3. The compensation pulses areshown to be uniform with each other, but that is only for illustration.In fact, they may have different shapes from each other.

As shown in FIG. 3, the general compensating pulses do not necessarilytake effect immediately after the pacing pulses. In the preferredembodiment, the compensating pulses start as quickly as possible afterthe pacing pulses, so as to better reverse the phenomena that areintended to be suppressed. The compensating pulses of the invention havedifferent preferred embodiments for long-term continuous pacing than foron-demand pacing. Indeed, the two scenaria are different.

Referring to FIG. 4, the preferred balanced charge, long-term continuouspacing waveform 20 is shown. One period of balanced charge waveform 20has a positive stimulating pulse 22 followed by a negative compensatingpulse 23. The positive stimulating pulse 22 has an amplitude 26 and aduration t1. The negative compensating pulse 24 has an amplitude 28, anda duration t2. As shown in FIG. 4, the magnitude of amplitude 28 forcompensating pulse 24 is less than the magnitude of amplitude 26 forstimulating pulse 22. It will also be observed that the compensatingpulses 24 start immediately at an end of a first pacing pulse 22, andlast up to an onset of a second pacing pulse 22 that succeeds the firstpacing pulse. This presents an advantage that artifacts are avoided.Indeed, it has further been found that any discontinuities in thecompensating current pulse will have a tendency to cause artifacts inthe monitored electrocardiogram (ECG) during pacing. The artifacts couldcause erroneous reading by a clinician monitoring the ECG. In this form,however, a nonzero current will always be applied to a patient over thecomplete period of the balanced charge waveform. Further, thecompensating pulses 24 may have substantially constant current. Thisreduces the overall instantaneous current applied to the patient fromthe compensating pulses, as is preferred.

Referring back to FIG. 1, the amount of positive charge applied by thestimulating pulse to the patient is equivalent to the amplitude of thepulse multiplied by the duration of the pulse, represented in FIG. 1 byan area 16 under the pulse. Referring to FIG. 4, in order for thewaveform to have a balanced charge, area 30 under the stimulating pulsemust be approximately equal to area 32 under the compensating pulse.Having equal areas ensures that the positive charge of the stimulatingpulse is balanced by the negative charge of the compensating pulse, andthe net current applied to a patient is approximately zero.

During on-demand pacing, however, a pacing pulse for stimulating thatpatient is generated only when an absence of a patient's heartbeat isdetected. Several stimulating pulses may therefore be regularly appliedfor a period of time, followed by a period when no stimulating pulsesare applied. As such, the balance charge waveform 20 shown in FIG. 4would be ideal for on-demand pacing only if it were guaranteed that thelast pacing pulse would be followed by a corresponding compensatingpulse, and then the waveform would end. If the pacing pulsesdiscontinued and then the compensating pulses continued indefinitely,the same effect would be accomplished as the prior art problem that theinvention solves.

Other waveforms according to the invention are now described, that arebetter suited for on-demand pacing. In those, a current of thecompensating pulses has a waveform of decay from an initially highvalue. The decay ends in a zero value. In some embodiments, as ispreferred, the decay completes to bring the pulse to zero before thenext pulse is applied, which ensures that the charge of the waveformremains balanced. In other embodiments the decay may be interrupted bythe next pacing pulse, but is brought to zero if no other pacing pulseis applied. These other embodiments are not preferred, however, becausethey would not result in an exactly balanced charge.

Referring to FIG. 5, an embodiment of the invention is shown where thedecay is exponential. A balanced charge waveform 40 includes a positivestimulating pulse 42 and negative compensating pulse 44. The stimulatingpulse 42 has an amplitude 46, and a duration t1. The compensating pulse44 has an initial non-zero amplitude 48, and a duration t2. In thepreferred embodiment, area 50 under positive stimulating pulse 42 isbalanced by area 52 under compensating pulse 44. The net charge appliedto a patient over one period of the balanced charge waveform is thuszero.

Referring to FIG. 6, an embodiment of the invention is shown where thedecay is linear. A balanced charge waveform 60 shown in FIG. 6 consistsof a stimulating pulse 62 followed by a compensating current pulse 64.As in all the balanced charge waveforms disclosed herein, the charge ofthe stimulating current pulse is equally balanced by the charge of thecompensating current pulse.

Those skilled in the art will recognize that there are many differentforms that the trailing edge of the compensating pulse can take. The tworepresentative forms disclosed herein are advantageous, in that they areeasily generated using capacitors as decay elements. Those skilled inthe art will recognize, however, that by digitizing the waveforms, thetrailing edge can take a variety of different shapes. As disclosed bythe method herein, however, two features of the trailing edge arepreferably embodied: (1) the amplitude of the compensating current pulseshould decay to nearly 0; and (2) the decay is relatively continuous,with few rapid changes in the amplitude, else artifacts might be causedin a monitored heart waveform.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.For example, even approximate balancing that only substantially (but notexactly) compensates is within the invention, and will result insubstantially alleviating the prior art problems.

1. A transcutaneous cardiac pacing method comprising: applying pacingelectrodes to a patient for transcutaneous cardiac pacing; applyingthrough the electrodes a series of cardiac pacing pulses to a heart ofthe patient; and applying through the electrodes a series ofcompensating pulses interleaved with the pacing pulses, the compensatingpulses having a polarity opposite to a polarity of the cardiac pacingpulses to at least substantially compensate for a charge of the cardiacpacing pulses and an amplitude less than an amplitude of the cardiacpacing pulses, the compensating pulses decaying from an initial,non-zero value to a second value that is substantially zero without adiscontinuity, and the cardiac pacing pulses and compensating pulsesbeing configured so that the net current applied to a patient issubstantially or exactly zero.
 2. The method of claim 1, in which atleast three of the compensating pulses alternate successively with atleast three of the pacing pulses.
 3. The method of claim 1, in which atleast one of the compensating pulses has substantially constant current.4. The method of claim 1, in which the decay is exponential.
 5. Themethod of claim 1, in which the decay is linear.
 6. A transcutaneouscardiac pacing method comprising: applying pacing electrodes to apatient for transcutaneous cardiac pacing; applying, through theelectrodes a series of cardiac pacing pulses to a heart of the patient;and applying through the electrodes a series of compensating pulsesinterleaved with the pacing pulses, the compensating pulses having apolarity opposite to a polarity of the cardiac pacing pulses to at leastsubstantially compensate for a charge of the cardiac pacing pulses andan amplitude less than an amplitude of the cardiac pacing pulses, thecardiac pacing pulses and compensating pulses being configured so thatthe net current applied to a patient is substantially or exactly zero,wherein a compensating pulse starts immediately at an end of a firstpacing pulse, and lasts up to an onset of a second pacing pulse thatsucceeds the first pacing pulse.
 7. The method of claim 6, in which atleast three of the compensating pulses alternate successively with atleast three of the pacing pulses.
 8. The method of claim 6, in which atleast one of the compensating pulses has substantially constant current.9. A system for transcutaneous cardiac pacing comprising: electrodes fortranscutaneous pacing; means for applying through the electrodes aseries of cardiac pacing pulses having a first polarity; and means forapplying through the electrodes a series of compensating pulsesinterleaved with the cardiac pacing pulses, the compensating pulseshaving a second polarity opposite to the first polarity and an amplitudeless than an amplitude of the cardiac pacing pulses, the compensatingpulses decaying from an initial, non-zero value to a second value thatis substantially zero without a discontinuity, and the compensatingpulses being configured to deliver an amount of charge of the secondpolarity substantially or exactly equal to the charge of the firstpolarity applied by the electrodes.
 10. The system of claim 9, in whichat least three of the compensating pulses alternate successively with atleast three of the pacing pulses.
 11. The system of claim 9, in which atleast one of the compensating pulses has substantially constant current.12. The system of claim 9, in which the decay is exponential.
 13. Thesystem of claim 9, in which the decay is linear.
 14. A system fortranscutaneous cardiac pacing comprising electrodes for transcutaneouspacing; means for applying through the electrodes a series of cardiacpacing pulses having a first polarity; and means for applying throughthe electrodes a series of compensating pulses interleaved with thecardiac pacing pulses, the compensating pulses having a second polarityopposite to the first polarity and an amplitude less than an amplitudeof the cardiac pacing pulses, and being configured to deliver an amountof charge of the second polarity substantially or exactly equal to thecharge of the first polarity applied by the electrodes, wherein acompensating pulse starts immediately at an end of a first pacing pulse,and lasts up to an onset of a second pacing pulse that succeeds thefirst pacing pulse.
 15. The system of claim 14, in which at least threeof the compensating pulses alternate successively with at least three ofthe pacing pulses.
 16. The system of claim 14, in which at least one ofthe compensating pulses has substantially constant current.